Higher cost from an unnecessary amount of concrete
used since the weight of the concrete,
when poured, will compress the ground below.

A compaction of at least 1/2 inch of the ground
when the concrete is poured.

Undesired water absorption at the bottom of the
concrete. Hence, e.g. 4-inch thick concrete
effectively reduces to 3-inch thick concrete
because the bottom 1-inch has become weak
and useless from lack of water in the cure process
(i.e. hydration process).